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Metal Casting Technologies : December 2007
METAL Casting Technologies December 2007 48 greensand mix in order to improve the surface finish of the casting, to prevent metal penetration, to minimize sand burn on, to reduce the chance of expansion defects, to improve casting strip on shakeout and to improve flowability of the system sand in transport and during moulding [7, 8]. According to the nature of the bentonite blend used these additions may also allow the system sand to operate at lower moisture levels and can improve green strength of the sand. Pulverized bituminous sea coal (coal dust) is still most popular carbonaceous addition but powdered forms of anthracite coal, metallurgical coke and lignite have also been used as supplements. Substitute materials called coal dust replacements, e.g. asphalt, bitumen and pitch are also widely available. During pouring of castings the organic compounds in the carbonaceous materials volatilize on heating as the liquid metal enters the mould. The atmosphere in the mould becomes reducing and is supersaturated with carbon. This volatilized carbon condenses on the mould surfaces as a film of lustrous carbon. The combination of this barrier film of lustrous carbon, the cushioning effect of the evolved volatiles, and the reducing atmosphere minimizes burn on and metal penetration. During heating the coal swells entering the pores between sand grains providing plasticized bonding so that the volume expansion of the sand grains, caused by structural transitions in SiO2, can be accommodated without fracture of the cast face mould surface, thus reducing the chance of expansion defects. 3. GREENSAND MOULDING IN THAILAND The production of ferrous castings using greensand moulding is a major part of the Thai foundry industry. As well as the many SME foundries producing a variety of iron and steel castings, there are a number of large iron foundries producing high volumes of grey (FC) and ductile (FCD) iron castings for the automotive industry. All foundries in hot countries like Thailand face problems in cooling the sand returning from shakeout before it returns to the muller. When ambient temperatures are around 35oC cooling return sand to below the desired maximum of 50oC is difficult unless the sand plant has large capacity. Above this temperature sand grains cannot be effectively coated with the bentonite bond during mulling resulting in variable sand properties and inconsistent mould performance. For example, the compactability of a sand can fall from 40-45% (at temperatures below 50oC) to 30-35% at 70oC . Most automotive foundries work at or near the maximum capacity of their sand plants due to the high production rates needed to satisfy their customers. Several automotive and other long run production iron foundries operate high pressure box-less moulding lines capable of turning out some 300 moulds per hour where the required operating windows for sand properties in terms of compactability, toughness, strength and permeability are very tight. In many Thai foundries sand related problems result in unacceptable casting quality reducing profitability. In the smaller foundries these problems can be significantly reduced by use of better raw materials and by improvements in sand plant equipment, in process control and in technical training. In the larger automotive foundries the plant is modern, technical skills are high and process control is effectively used, however high production rates and customer demands (zero defects, minimum variation, cost down, etc.) present exacting challenges to sand technologists and to raw materials suppliers seeking to improve greensand performance. There is also a need for research into making use of the increasing amounts of spent foundry sands and other "waste" foundry materials rather than send them for land fill disposal at waste dumps. 4. MAKING GREENSAND GREENER Environmental improvements must also be made. As well as tackling dust issues and the re-use of spent sand foundries have to reduce the volatiles and other emissions generated by the decomposition of the lustrous carbon forming additives, mould release agents and mould coatings, and by the breakdown of any chemically bonded cores that may be present in the moulds. The decomposition of mould and core constituents, and the origins, nature and composition of the cocktail of gases emitted during and after pouring have to be characterized and understood if improvements are to be made. Considerable progress has been made in Europe and the US in reducing the levels of volatile organic compound (VOC) emissions, such as benzene and toluene. Under the Castings Emissions Reduction Program (CERP)  significant research on emissions from Greensand moulds has been carried out in the US [1, 2, 10-12]. The major areas of investigation have included: ■ Study and measurement of emissions generated by decomposition of additives to Greensand ■ Effects of sea coal particle size on emissions ■ Application of advanced analytical techniques in characterization of emission constituents